A comparative study of the mechanical properties of single intact skeletal muscle fibres isolated from wild-type and MLC/mIgf-1 mice.

The expression of the mIgf-1 transgene is selectively restricted to skeletal muscle and is especially expressed in fast fibres. The increase in muscle mass is considered a “functional hypertrophy” (Musarò et al., 2001). To improve the knowledge of the mechanical properties of this murine model we made a comparative study between wild-type (WT) and MLC/mIgf-1 (TG) mice using single intact fibres dissected from the flexor digitorum brevis muscle. In addition to the “classical” parameter we analyzed the “static stiffness” (SS), a non-crossbridge calcium-dependent stiffness previously identified in activated frog muscle fibres (Bagni et al., 2002). The fibres were mounted in an experimental chamber (~23°C) between a force transducer and an electromagnetic motor. Sarcomere length was measured by means of a videocamera. A preliminary analysis of the results shows that: 1) tetanic tension and force-velocity relation in WT and TG mice were not significantly different; 2) the plateau of length-tension relation is in agreement with the length of the myofilaments reported in literature; 3) TG fibres exhibited an increase in diameter and maximum force, but specific force was the same as for WT fibres; 4) SS was present either in WT or in TG fibres, but its characteristics were slightly different in the two preparations which suggests mechanical differences or different calcium response of the structure responsible for the static stiffness. Our experiments show that single intact fibres could be used for mechanical studies of different murine models under various conditions such as diseases or aging.